reactor: Main class to adjust visivility of fields

[miurahr]

Pull request type

Please mark github LABEL of the type of change your PR introduces:

• Other (describe below)
• refactor

Which ticket is resolved?

What does this PR change?

• Adjust for the following SpotBugs warnings.

M V MS_MUTABLE_COLLECTION_PKGPROTECT MS: org.omegat.Main.PARAMS is a mutable collection which should be package protected At Main.java:[line 136]	
M V MS_PKGPROTECT MS: org.omegat.Main.projectLocation should be package protected At Main.java:[line 130]	
M V MS_PKGPROTECT MS: org.omegat.Main.remoteProject should be package protected At Main.java:[line 133]	
M V MS_PKGPROTECT MS: org.omegat.Main.runMode should be package protected At Main.java:[line 139]	
M D NP_NULL_ON_SOME_PATH_FROM_RETURN_VALUE NP: Possible null pointer dereference in org.omegat.Main.restartGUI(String) due to return value of called method Dereferenced at Main.java:[line 267]	
M D REC_CATCH_EXCEPTION REC: Exception is caught when Exception is not thrown in org.omegat.Main.runGUI() At Main.java:[line 358]	

Other information

#1296